Solar cells directly convert inexhaustible and clean pollution-free solar energy into electrical energy, and hence they can be said to be important key devices in view of the environmental and energy exhaustion problems.
In general, a solar cell comprises a light incident surface side electrode arranged on the side where solar light enters, a counter electrode, and a semiconductor photoelectric conversion layer sandwiched between the electrodes. The photoelectric conversion layer now industrially produced is commonly made of silicon (Si), and the solar cell using Si normally includes a PN or PIN junction of monocrystalline Si, polycrystalline Si or amorphous Si (hereinafter, often referred to as “a-Si”). Besides that, there are also practical solar cells using compound semiconductors such as GaAs and chalcopyrite. The light incident-side electrodes adopted in many solar cells are comb-shaped metal electrodes, which are called “finger electrodes”. However, solar cells using semiconductors having large surface resistivity, such as solar cells of a-Si type, are often equipped with not finger electrodes but transparent electroconductive films as the light incident-side electrodes.
At present, the largest problem of solar cells is to increase the photoelectric conversion efficiency. The photoelectric conversion efficiency of solar cells is generally in the range of about 10 to 15%. In order to increase the conversion efficiency, various improvements have been hitherto made. Those improvements are, for example, in that an antireflection film is formed and/or the light receiving surface is made to have a texture structure so as to reduce the reflection loss and in that a getter layer or a surface passivation film is provided so as to prevent the carrier recombination in the bulk or on the surface. Further, the improvements particularly for enhancing the light-receiving efficiency are, for example, in that the semiconductor layer is thickened and/or made of materials having large light-absorption coefficients and in that the effective incident area is enlarged by adopting an embedded electrode or a back electrode type solar cell.
It is also studied to improve the electrode structure for the sake of increasing the light transmittance and/or the conversion efficiency.
Those prior improvements, which, for example, aim at enlarging the effective incident area, are mainly for the purpose of increasing the light transmittance, and hence they by no means increase the conversion efficiency of the absorbed solar light for carrier excitation. The conversion efficiency, therefore, is not significantly improved.